The present invention relates to a snubber circuit for protecting self-arc-extinguishing type power semiconductor elements forming a power converter such as an inverter, and more particularly to a simplified snubber circuit for a power converter.
FIG. 4 shows a circuit diagram of a typical power converter. The same or corresponding parts have been labelled with the same reference numerals in subsequent drawings. This circuit is used in an inverter for driving a motor, and in an uninterruptive power supply and the like. The diagram illustrates a circuit in which self turn off semiconductor elements such as power transistors are used as self-arc-extinguishing type power semiconductor elements. In FIG. 4, free-wheeling diodes 4 (41, 42) are usually connected in parallel to power transistors 3 (31, 32), respectively, and the transistor 32 is in a non-conducting state when the transistor 31 is conducting. Under such a state, an electric current is applied to an inductance 8 and a resistance 9 forming a load from a d.c. power supply 1 through the transistor 31. When the transistor 31 turns OFF under this state, the electric current which has been applied t the transistor 31 is coupled to the free-wheeling diode 42 and the load current is kept constant. At this time, charge and discharge type R-C-D snubber circuits 10 and a clamp type R-C-D snubber circuit 11 such as shown in FIG. 4 are connected for the purpose of reducing a surge voltage generated by energy stored in a floating inductance 2 by means of wirings from the capacitor of the d.c. power supply 1 to the transistors 3.
With further reference to those snubber circuits, the charge and discharge type snubber circuits 10 connected in parallel to the transistors 3 are used for the purpose of controlling dv/dt so as to reduce the turn-off duty of the elements, and the clamp type snubber circuit 11 connected across both ends of the d.c. power supply 1 is used for the purpose of absorbing energy stored in the floating inductance 2 of a power supply feeder line. Sometimes a driving circuit of the semiconductor element 3 or a control circuit of the power converter malfunctions due to an excessive vibration or oscillation voltage generated at the time of reverse recovery of snubber diodes SD in these snubber circuits 10 and 11. For the purpose of preventing such malfunctions from occurring, R-C snubbers 12, each consisting of a series circuit of a resistor and a capacitor, are connected to snubber diodes SD as shown in FIG. 4.
In the power converter shown in FIG. 4, however, the snubber circuit is complicated and expensive as a result of the numerous components required.
With the spread in the use of inverters it is an object of the present invention to reduce the necessary components further for the purpose of cost reduction and improvement of reliability of the inverter.
It is an object of the present invention to simplify the composition of a snubber circuit attached to a self-arc-extinguishing type power semiconductor element so as to reduce the quantity of components and to form a more compact power converter.